1. Field of the Invention
The invention relates to chemical indicators.
2. Description of Related Art
Chemical indicators are used in many processes as a diagnostic tool to indicate whether a desired process or reaction has taken place. One important use of chemical indicators is in the medical industry in sterilization systems to visually indicate whether a sterilization process has taken place. For example, many medical instruments must be sterilized prior to use on a patient. Thus, instrument manufacturers often sterilize instruments before providing them to hospitals, and hospitals generally have their own sterilization systems to sterilize instruments prior to use on a patient. One popular type of sterilizer is the STERRAD® brand of sterilizers manufactured by Advanced Sterilization Products, of Irvine, Calif., a division of Ethicon, Inc. of Somerville, N.J.
The STERRAD® sterilizers are designed to house one or more removable trays that fit within a sterilization chamber. Each tray may be filled with medical instruments such as scalpels, endoscopes, scissors, and the like. To sterilize the medical instruments, the sealed chamber is exposed to a vacuum and hydrogen peroxide or other oxidizing agent vapor is introduced into the chamber. Hydrogen peroxide is a popular oxidizing agent because of its potent sporicidal effect as well as its ability to decompose readily into water and oxygen after the sterilization process making the sterilization chamber safe for human contact.
As a way to demonstrate that a sterilant such as hydrogen peroxide is present in a sterilization chamber of a sterilizer (e.g., a STERRAD® sterilizer), a chemical indicator that is sensitive to the sterilant is placed in the chamber along with the items to be sterilized. A popular form of chemical indicator is a strip of paper or other substrate with a dye adhered to one side of the substrate. A visible change in the dye of the chemical indicator, such as a color change, indicates that the items contained within the chamber have been exposed to the sterilant.
Previous chemical indicators for sterilization processes such as described rely upon dyes that are sensitive to changes in pH. Exposure to an oxidizing sterilant (e.g., hydrogen peroxide) generally changes the pH in the sterilization chamber from basic to acidic. The dye of the prior art chemical indicators are generally acid-base indicators and change color as the pH of the system changes.
One problem with prior art chemical indicators used in the manner described is that the dye is generally not permanently changed after the sterilization process and thus the color change may be reversed under certain conditions such as exposure to a basic environment. For example, a popular pH-sensitive dye is a phenol red dye adhered to a paper substrate. The phenol red dye changes from a red to yellow color when the chemical indicator is exposed to an acidic environment such as a hydrogen peroxide sterilant. It has been found, however, that the color change could be reversed under exposure to a basic environment.
A further problem with prior art chemical indicators such as the phenol red chemical indicator, is that the indicator is generally not specific, for example, to the oxidant for which it is to be used. Instead, the phenol red chemical indicator will change color in response to a change in pH brought about by any agent, including environmental factors. Some factors that may cause a premature or undesired color change include ambient light, instability of the chemical formulation, and environmental contaminants. Thus, the phenol red chemical indicator is often combined with other agents, such as ultraviolet light stabilizers or hydrophilic agents to resist undesired changes in color. The non-specificity of the phenol red chemical indicator yields stability problems both in the storage of the chemical indicator as well as evidencing a prior test result.
What is needed is a chemical indicator, particularly for oxidation-type sterilization processes that offers a non-reversible color change and is chemically stable.